In an optical head device which performs recording and reproduction of information in and from an optical recording medium such as a CD or a DVD, a laser beam emitted from a semiconductor laser is converged on the recording face of the CD or the DVD through an objective lens. Conventionally, a refractive lens in a simple aspheric surface shape that is molded with resin or a lens that is formed with a diffraction pattern on its refraction face is used as the objective lens.
However, a refractive lens made of resin is commonly designed and produced such that its aberration is minimized near ordinary room temperature and thus the spherical aberration varies when an environmental temperature changes to cause the refractive index of the lens to vary or the expansion and shrinkage of the lens to occur. Therefore, jitter is deteriorated because a large spherical aberration occurs under a low-temperature environmental or a high-temperature environment. A lens made of resin is used because of cost savings even though the variation of refractive index and the expansion and shrinkage are larger than those of a glass lens. Therefore, when a lens made of resin is used, a lens with a small numerical aperture “NA” is used in order to minimize the variation of aberration while sacrificing high speed. Alternatively, a compromise in the range of its operating temperature is required.
Further, in a lens whose refraction face is provided with a diffraction pattern, a method has been proposed in which the variation of spherical aberration is cancelled by utilizing the variation of diffraction force due to the variation of wavelength. However, since a minute diffraction pattern is required to be formed in such a lens, a molding die is required to be produced by a special device.